Source

File: wp-admin/includes/class-wp-filesystem-ftpext.php

public function get_contents( $file ) {
		$tempfile   = wp_tempnam( $file );
		$temphandle = fopen( $tempfile, 'w+' );

		if ( ! $temphandle ) {
			unlink( $tempfile );
			return false;
		}

		if ( ! ftp_fget( $this->link, $temphandle, $file, FTP_BINARY ) ) {
			fclose( $temphandle );
			unlink( $tempfile );
			return false;
		}

		fseek( $temphandle, 0 ); // Skip back to the start of the file being written to.
		$contents = '';

		while ( ! feof( $temphandle ) ) {
			$contents .= fread( $temphandle, 8 * KB_IN_BYTES );
		}

		fclose( $temphandle );
		unlink( $tempfile );

		return $contents;
	}